Cells often experience conditions during which the workload on the endoplasmic reticulum (“ER”) protein folding machinery exceeds its capability. Such cells are said to be experiencing “ER stress.” ER stress can result from secretory work overload, expression of folding-defective secretory proteins, deprivation of nutrients or oxygen, changes in luminal calcium concentration, and deviation from resting redox state. Sophisticated cellular surveillance and quality control systems work to maintain ER homeostasis under such perturbations. Under ER stress, secretory proteins accumulate in unfolded forms within the organelle to trigger a set of intracellular signaling pathways called the unfolded protein response (UPR). UPR signaling increases transcription of genes encoding chaperones, oxidoreductases, lipid-biosynthetic enzymes, and ER-associated degradation (ERAD) components (Travers, K. J. et al. Cell 101, 249-258 (2000)).
In some instances, the ER stressed state remains too great, and cannot be remedied through the UPR's homeostatic outputs. In these situations, the UPR switches strategies and actively triggers apoptosis (Zhang, K. & Kaufman, R. J. Neurology 66, S102-109 (2006)); we have named this destructive signaling state the Terminal UPR (signature events of the Terminal UPR are described herein). Apoptosis of irremediably stressed cells is an extreme, yet definitive, quality control strategy that protects multicellular organisms from exposure to immature and damaged secretory proteins. So at the cost of losing some cells, multicellular organisms may benefit temporarily from Terminal UPR-induced apoptosis. However, many deadly human diseases occur if too many cells die through this process. Conversely, many human diseases such as diabetes mellitus and retinopathies proceed from unchecked cell degeneration under ER stress (Merksamer, P. I., and Papa, F. R., J Cell Sci 123, 1003-1006 (2010); Papa, F. R. Cold Spring Harbor perspectives in medicine 2, a007666 (2012); Shore, G. C., Papa, F. R., and Oakes, S. A., Curr Opin Cell Biol 23, 143-149 (2011)). Type 2 diabetes may be a prototype of cell degenerative diseases caused by UPR-mediated apoptosis under irremediable ER stress. These same principles appear to be at play in type 1 diabetes, wherein immune attack on islet β-cells elevates ER workload and causes ER stress in remaining cells. A deeper fundamental and mechanistic understanding of the etiology and pathogenesis of diabetes mellitus may lead to increasing opportunities for the development of novel and effective therapies. Terminal UPR signaling is central to these conditions as shown through experimental data and uses of proprietary compounds that defeat the consequences of terminal UPR signaling in ER stress-challenged β-cells to afford significant cytoprotection.
IRE1α and IRE1β are ER-transmembrane proteins that become activated when unfolded proteins accumulate within the organelle. IRE1α is the more widely expressed and well-studied family member. The bifunctional kinase/endoribonuclease IRE1α controls entry into the terminal UPR. IRE1α senses unfolded proteins through an ER lumenal domain that becomes oligomerized during stress (Zhou, J. et al. Proceedings of the National Academy of Sciences of the United States of America 103, 14343-14348 (2006); Credle, J. J. et al. Proc Natl Acad Sci USA 102, 18773-18784 (2005); Aragon, T. et al. Nature (2008); Aragon, T. et al. Nature 457, 736-740 (2009)). On its cytosolic face, IRE1α possesses bifunctional kinase/RNase activities. Oligomerization juxtaposes IRE1α's kinase domains, which consequently trans-autophosphorylate. Kinase autophosphorylation activates the RNase activity, which cleaves XBP1 mRNA at specific sites to excise an intron. Religation of IRE1α-cleaved XBP1 mRNA shifts the open reading frame; translation of spliced XBP1 mRNA produces a transcription factor called XBP1s (s=spliced) (Calfon, M. et al. Nature 415, 92-96, (2002); Yoshida, H. Cell 107, 881-891 (2001)). XBP1s's target genes encode products that enhance ER protein folding and quality control (Lee, A. H. et al., Molecular and cellular biology 23, 7448-7459 (2003)). Thus, IRE1α promotes adaptation via XBP1s.
Under irremediable ER stress, positive feedback signals emanate from the UPR and become integrated and amplified at key nodes to trigger apoptosis. IRE1α is a key initiator of these pro-apoptotic signals. IRE1α employs auto-phosphorylation as a “timer.” Remediable ER stress causes low-level, transient auto-phosphorylation that confines RNase activity to XBP1 mRNA splicing. However, sustained kinase autophosphorylation causes IRE1α's RNase to acquire relaxed specificity, causing it to endonucleolytically degrade thousands of ER-localized mRNAs in close proximity to IRE1α (Han, D. et al. Cell 138, 562-575, (2009); Hollien, J. et al. Journal of Cell Biology. These mRNAs encode secretory proteins being co-translationally translocated (e.g., insulin in β cells). As mRNA degradation continues, transcripts encoding ER-resident enzymes also become depleted, thus destabilizing the entire ER protein-folding machinery. Once IRE1α's RNase becomes hyperactive, adaptive signaling through XBP1 splicing becomes eclipsed by ER mRNA destruction, which pushes cells into apoptosis.
A terminal UPR signature tightly controlled by IRE1α's hyperactive RNase activity causes (1) widespread mRNA degradation at the ER membrane that leads to mitochondrial apoptosis (Han, D. et al. Cell 138, 562-575, (2009)), (2) induction of the pro-oxidant thioredoxin-interacting protein (TXNIP), which activates the NLRP3 inflammasome to produce maturation and secretion of interleukin-1β, and consequent sterile inflammation in pancreatic islets leading to diabetes (Lerner, A. G. et al. Cell metabolism 16, 250-264, (2012)), and (3) degradation of pre-miRNA 17, leading to translational upregulation and cleavage of pre-mitochondrial caspase 2 (Upton, J. P. et al. Science 338, 818-822, (2012)) and stabilization of the mRNA encoding TXNIP (Lerner, A. G. et al. Cell metabolism 16, 250-264, (2012)).
Retinitis pigmentosa (RP) is a clinically and genetically heterogeneous group of inherited retinal disorders characterized by diffuse progressive dysfunction and loss of rod and cone photoreceptors, and retinal pigment epithelium. There are no approved therapies to offer the over 100,000 Americans who currently suffer from RP. As RP is a leading cause of irreversible vision loss, new therapeutic approaches for this condition would be expected to have significant cost-saving benefits for health care systems.
A great deal of evidence suggests that the accumulation of misfolded proteins within the ER is a central causative mechanism in many forms of RP. When the protein-folding capacity of the ER is overwhelmed, cells experience “ER stress” and actively commit programmed cell death. For example, mutations in rhodopsin are the most common cause of RP in the US and lead to a defective rhodopsin protein that misfolds and accumulates in the ER to cause high levels ER stress.
Disclosed herein, inter alia, are solutions to these and other problems in the art.